[filmscanners] RE: Density vs Dynamic range

["Austin Franklin" <darkroom@ix.netcom.com>]

Anthony,

> This all seems to be a lot of vast overcomplication.  Dynamic range is the
> highest signal divided by the lowest signal.

It isn't that simple, if you understand it.  What you say above may or may
not be true, it depends on what your units are, and what you mean by highest
signal and lowest signal.  Dynamic range takes into account the absolute
range of the signal AND noise.  If by lowest signal you mean the lowest
DISCERNABLE signal (noise), and you are not in log form, then you are right.

> Density range is the highest
> density divided by the lowest density.

Well, since density is measured in log form, the absolute density range (as
opposed to density range, which would simply be stated as lowest value to
highest value) would be highest - lowest, because in log notation,
subtraction is the same as division in non-log notation.

> The only difference
> between the two
> is that dynamic range is additive, like primary colors in a CRT
> display, and
> density is subtractive, like the subtractive primaries in a slide (cyan,
> yellow, magenta).

I have no idea what you are trying to say here...I see no sense in it.  I've
never seen anyone make anywhere NEAR that statement about dynamic range.

> So what is the dynamic range of a slide?  It doesn't have one; it has a
> density range.  It only has a dynamic range if you shine light through it,
> and then the dynamic range depends on the light source.

Everything has a dynamic range.  It's simply a matter of finding/defining
the noise source (and everything has noise) and the overall range.  An
example of noise in the slide is film grain/dye cloud noise...the ability of
the grain/cloud to represent a color accurately.  Even prints have dynamic
range.

> With no light
> shining through the slide, its dynamic range is zero.  With light shining
> through it, the dynamic range is equal to the luminosity of the brightest
> part of the slide divided by the luminosity of the darkest part of the
> slide; in theory, that would always be the same, but in practice, due to
> other factors like stray light, it may vary depending on the situation.

What you are talking about is density range, not dynamic range.  But, even
with no light, the slide still has a density range...even if no one is
looking at it.  Kind of like a tree falling in the forest does it make a
sound even if no one hears it.

> So when you scan, what is the dynamic range and density range?
> They end up
> being the same thing.

Absolutely not correct.  You can scan something that has a very high density
range, with a scanner that has little dynamic range.  You appear to have the
common misconception of what dynamic range really is, and believe it's the
same as density range, which it is not.

<other "stuff" snipped>

Austin

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